Previously known reference apparatus require an external heading reference, usually magnetic, to establish and maintain heading, the accuracy being limited to that of the external source. Prior art includes:
(1) A platform is supported on gimbals relative to the vehicle, and the platform is held locally level by signals from gyroscopes and accelerometers.
(2) A single gyroscope is used and is suspended on gimbals with its spin axis vertical or horizontal. It is held in place, and its output signals are used to produce usable signals.
(3) Angular rates are supplied indirectly through gimbal resolvers.